5.1 Work and Power
Objectives:
Describe the conditions which
must be met to do work.
Distinguish between work and
power.
Calculate work and power.
Interpret data from a sample
electric bill.
5.1 Work and Power
MAKE A LIST OF FIVE ACTIVITIES
THAT YOU CONSIDER WORK:
1.
2.
3.
4.
5.
Work
 In science the
word ‘work’ relates to
forces, motion and
energy.
More specifically
work is a force acting
on an object in the
direction in which it
moves.
When you rake leaves,
you do work.
Work
There are two
conditions that must
be met in order for
work to be done:
 The object must
move.
 A force must act
on the object in the
direction the object
moves.
When you rake leaves,
you do work.
Energy and Work
Energy is needed to
rake leaves or to do
any kind of work.
In fact, energy is
defined as the
ability to do work.
When you run laps around
a track you do work. Your
legs apply a force against
the track to move your
body forward.
For work to be
done, a force must
move an object
through a distance.
Measuring Work
To determine the amount of work done, you use a
mathematical formula.
The formula relates work to force and distance.
Two measurements are needed to use this formula.
1. The amount of force exerted in the direction
of motion.
2. The distance moved.
Write the equation
Unit of Measurements
The amount of force
exerted on an object is
measured in Newtons.
The distance the
object moves is
measured in meters.
The downward force does
not do work. Only the force
applied in the same direction
the lawn mower moves does
work.
As a result, the unit
for work is sometimes
called the newton
meter (Nm) or joule.
Unit of Measurement - Joules
One joule (J) equals the
work done by a force of 1
N that moves an object a
distance of 1 m.
The joule is a unit that is
also used to measure
energy.
As you can see, energy &
work are related.
Lifting a glass of water from
a counter top to your mouth,
for example, would require
you to use about 1 J of
energy and to perform about
1 J of work.
SAMPLE PROBLEMS
How much work is done if I apply a force of 5N
to move an object 2 meters across a desk?
W=F*d
W=(5)(2)
W= 10J
How much energy was used to move it?
10J of energy
SAMPLE PROBLEMS
I mowed my lawn this weekend and I used 15 N to mow my lawn a
distance of 10 m – how much work did I do?
W=(15)(10)
W= 150J
How much work did I do if I used a force of 30 N to pull a table a
distance of 3 m?
W=(30)(3)
W= 90J
My two kids worked together to move a heavy ladder to the garage a
distance of 2 m. They used a combined force of 25 N to complete
the task. How much work did they do together?
W=(25)(2)
W=50J
DO Practice Problems 1-5 pg. 110
Power
 Power is the rate at which work is done.
 To calculate power, you divide the amount of
work done by the time it took to do the work.
Write the equation:
 When the work is in joules and the time in
seconds, the power is in joules per second (J/s).
 Another name for a joule per second is the
watt (W).
Watt (W).
One watt is equal to 1 J/s.
The SI unit of power was
named for James Watt, a
Scottish engineer.
Watt coined the term
"horsepower“ (hp), which is
the amount of work a horse
could do in one second.
One horsepower is equal to
745.56 W
WATT
One watt is about the
amount of power it takes for
you to lift one glass of
water one meter in one
second.
SAMPLE PROBLEMS
If I run up several flights of stairs in 1.5 min.,
and the work I do is equal to 9000J, how much
power did I use.
P= work (J)/time (s)
p=(9000)/(1.5)(60)
p=(9000)/(90)
p=100J/s or 100 W (watts)
SAMPLE PROBLEMS
I decided to go rowing this weekend. I rowed
the boat for 14 min. And did a 168,000 J of
work, how much power did I exert?
P=168000/(14)(60)
P=168000/(840)
P=200 W
DO Practice Problems 7-9 pg. 110
CHECK & EXPLAIN pg.111
1. No; even though the push required effort, by
definition work is done only if an object moves.
2. Both are measurements that relate force and
distance. Power is the rate at which work is done
so it includes the measurement of time.
3. Work=500N X 1.5 m = 750 J
Power= 750J/2s=375 W
4. The charge for the electricity usage is
$63.43.
5.2 Work and Machines
Objectives
Explain how machines make work
easier.
Calculate mechanical advantage.
Infer the relationship between energy
use and mechanical efficiency.
Machines
A machine is a
device that makes
work easier.
Machines make
work easier by
changing the
direction or the
size of the force
needed to do work.
Machines and Forces
Two forces are involved when you use a
machine.
The force applied
to the machine is
called the effort
force .
When you push
down on a
screwdriver to
remove the lid,
you apply effort
force to the
screwdriver.
The force opposing
the effort force is
called the resistant
force.
The lid on the
paint can is the
resistance, or
opposing, force.
Often resistance
is the weight of
the object.
Mechanical Advantage
Most machines multiply the
force of your efforts.
The number of times a
machine multiplies an effort
force is its mechanical
advantage (M.A.).
A machine with an M.A. of 2
doubles your effort force.
As a result, you only have to
use half of the effort force
needed to do the same
amount of work without a
machine.
Mechanical Advantage
To find out a machine's mechanical
advantage, divide the resistance force by
the effort force.
* For example, if you can lift a 300 N object
by applying only 20 N of force to a lever, the
M.A. of the lever is 15.
SAMPLE PROBLEMS
John likes to work on old automobiles. He has
constructed a pulley system to help her lift the
engine out of the car. She can lift a 600 N
engine by applying only 150 N of force. What is
the M.A. of her pulley system?
M.A. =resistance force/effort force
M.A. = 600 N/150 N
M.A. = 4
Do problem 1-3 0n pg. 113
Mechanical Efficiency
The amount of work put into a machine, or work
input, is always greater than the amount of work
done by the machine, or work output because
some of the work put into the machine is used to
overcome friction.
The mechanical efficiency of a machine compares
its work output with the work input. In general:
Because the work output is always less than the work input,
the mechanical efficiency of a machine is always less than
100%.
5.3 Simple and Compound Machines
Objectives
Explain how six simple machines make
work easier.
Classify the simple machines in a
compound machine you are familiar with.
Predict the mechanical advantages of
simple machines.
SIMPLE MACHINES
Machines are almost everywhere you look.
Below are two groups of devices.
Group 1: Ramp, bottle opener, pulley, wheelbarrow
Group 2: Car, escalator, lawn mower, hair dryer
Which group of items contains machines?
All devices in both groups make work easier.
Each changes the size or direction of a force applied to
it.
The devices in Group 1 are examples of simple machines.
Simple machines do work with one movement
SIMPLE MACHINES
There are six types of simple machines:
the inclined plane, wedge, screw, lever,
wheel and axle and pulley.
Inclined Plane
A simple machine
that has a sloping
surface is an
inclined plane.
A ramp makes moving a heavy crate
easier.
A wedge is an inclined plane
that can move.
An ax is a
wedge.
A screw is also an inclined plane.
Many car jacks
are screws.
Levers
A balance, a wheelbarrow, and a shovel are all
machines that move when a force is applied.
They all have a straight point that moves when a
force is applied & one point that does not move,
called a fulcrum.
Machines that do work by moving around a fixed
point are called levers.
Shovel
Wheelbarrow
Balance
TYPES OF LEVERS - First Class Lever
First class levers multiply the effort force and also
change its direction.
•The fulcrum is
always between
the effort
force and the
resistance
force.
TYPES OF LEVERS - Second Class Lever
A second class lever is defined as having the
resistance force between the fulcrum and the effort
force.
• Second class levers
multiply the effort
force without changing
its direction.
•A wheelbarrow and
nut cracker are
examples of a second
class lever.
•
TYPES OF LEVERS -Third Class Lever
A third class lever is defined as having the effort
force is between the fulcrum and the resistance
force.
•When you use a third class
lever, the effort force is
greater than the resistance
force.
• The reason for using a third
class lever is to increase the
distance moved, not to reduce
the force.
• Your forearm is an example
of a third class lever.
M.A. of Levers
The mechanical advantage of a lever is calculated by
dividing the effort distance by the resistance
distance.
• The M.A. of first and second class levers is
usually greater than 1.
• The M.A. of third class levers is less than 1.
• Third class levers do not multiply force.
Wheel and Axle
This type of simple
machine consists of
two circular objects
called a wheel and an
axle.
An effort force applied at the
wheel is multiplied at the axle to
overcome a resistance force.
The effort force applied to the
wheel moves over a greater
distance than the resistance
force does.
The wheel has a
larger radius than
the axle does.
The radius is the
distance from the
center of the wheel
to the edge.
M.A. of Wheel and Axle
The mechanical advantage of a wheel and axle is equal
to the radius of the wheel divided by the radius of
the axle.
• The M.A. of a wheel and axle is always greater
than 1.
Pulleys
A pulley is a
rope wrapped
around a grooved
wheel.
The two main
types of pulleys
are called fixed
pulleys &
movable pulleys.
• A fixed pulley is
attached to a
stationary structure.
• It can make lifting
an object easier by
changing the
direction of the
effort force.
•The M.A. of a
fixed pulley is one
because it does not
multiply the effort
force.
Pulleys
A movable pulley is hung on a rope and hooked to
a resistance.
• Movable pulleys can
multiply your effort
force.
• They have a M.A. of 2
• Notice that this pulley
system also changes the
direction of force.
When two or
more pulleys
are used
together, a
pulley system
is formed.
Pulleys
This pulley system below has a M.A. of 3.
The mechanical advantage
of a pulley system is equal
to the number of rope
segments pulling up on the
resistance force.
http://www.historyforkid
s.org/scienceforkids/phys
ics/machines/pulley.htm
Compound Machines
A compound machine is a
system of two or more
simple machines.
The mechanical advantage
of a compound machine is
much greater than that of
a simple machine.
The combination of simple
machines in the compound
machine multiplies the
total mechanical advantage.
How many simple machines
are there in this pencil
sharpener?
CHECK & EXPLAIN pg.121
Answer 1 & 4
1. Inclined plane - You use less force to pull an object up an inclined plane,
but go a greater distance. It reduces the effort force.
wedge – an inclined plane that can move.
screw – an inclined plane that produces a far greater force than
force needed to turn the screw.
the
lever – a machines that does work by moving around a fixed point.
wheel & axle – a machine consisting of a wheel and axle. The effort force
applied at the wheel moves over a greater distance than the resistance
force does.
pulley - a machine that makes lifting an object easier by changing the
direction of the effort force .
4.
The M.A. is 4.
Even though you cut down on the effort needed to lift something., you
now have to increase the distance you have to pull the rope. In other
words, if you use four pulleys, it takes 1/4 the effort to lift something,
but the distance you have to pull the rope is four times as far.
5.4 Energy and Its Forms
Objectives
Name and describe five forms of
energy.
Give examples of energy conversions.
Describe the Law of Conservation of
Mass and Energy.
Infer the role of energy conversions
in an everyday situation.
Machines, Work & Energy
Machines can make your work easier, but machines
can't save work or energy. The amount of work
you get out of a machine can never be greater
than the amount of work or energy put into it.
For example, the work a lawn mower engine does is
less than the energy in the gasoline that is
burned. Most of the energy is converted to heat,
due to friction between moving parts in the motor.
Remember, work and energy are related.
Energy is the ability to do work.
FORMS OF ENERGY
Energy has many forms. In some way, all of
these forms are related to each other. Energy
can be converted from one form to another.
Mechanical Energy
Mechanical energy is
the energy of the
movement or the
position of an object.
The mechanical energy
of a moving object is
also called its kinetic
energy.
The mechanical energy
of an object due to its
position is also called
potential energy.
A windmill is a compound machine that
uses mechanical energy to do work.
Nuclear Energy
All matter is made up
of particles called
atoms.
A great amount of
energy is stored in an
atom's core, or nucleus.
The sun has great amounts of
nuclear, heat, and
electromagnetic energy.
This energy, called
nuclear energy, is
released when an
atomic nucleus breaks
apart, or when a new
nucleus forms.
Chemical Energy
Chemical bonds
between atoms hold
substances
together.
Breaking these
bonds releases
large amounts of
energy.
Chemical energy is
the energy stored
in the bonds
between atoms.
Energy released from gasoline
drives the piston that moves the
car wheels.
Electromagnetic Energy
When electrons flow through conducting
material, a moving charge is formed.
This moving charge is electricity, a form of
electromagnetic energy.
http://www.jea.com/community/education/
electric/how.asp
Heat Energy
When a substance becomes
warmer, its particles move
faster.
The fast-moving particles
of air collide with the
slower moving water
particles, increasing their
motion.
Energy flows, as heat,
from the warm air to the
cool water.
Energy Conversions
How does energy change from one form to
another as you light the match?
When you move the match head against the
striking strip, particles in the match head
begin to move faster.
As this happens, mechanical energy changes
to heat energy.
The heat energy causes a chemical change
to begin.
The match head begins to change color and
release stored chemical energy.
The chemical energy changes into heat
energy and light energy.
The Law of Conservation of Mass and Energy
As energy changes from one
form to another, it is never
created or destroyed.
This fact has been observed
to be true in so many
different situations that it
has become a scientific law.
Albert Einstein predicted the
relationship between matter
and energy. He expressed it
in mathematical form as:
According to the Law of
Conservation of Mass and
Energy, the amount of mass
and energy in the universe
is always the same.
CHECK & EXPLAIN pg.125
Answer 1, 2 & 3
1. Mechanical energy – mixer; nuclear energy – reactors in a
nuclear power plant; chemical energy – combustion of
gasoline; electromagnetic energy – electricity; heat –
energy used tin cooking.
2. a. chemical energy stored in the gasoline is converted to
heat and mechanical energy in the engine.
b. chemical energy released from food by the body is
converted into heat energy used in running.
3. Energy changes from one form to another, it is never
created or destroyed.